Vibration is a problem in helicopters. An oscillatory hub sheer is produced at the rotor hub of the lift unit of a two blade helicopter, for example, with a frequency that is three times that of the revolution rate of the rotor, i.e., three per revolution. Although the hub shear oscillation is transverse to the axis of the rotor, this force is transmitted to the fuselage of the helicopter as a moment about the center of the gravity of the rotor. This moment in turn produces vertical vibrations within the helicopter fuselage. The intensity of the vibrations in a helicopter is a vital factor in determining the lifetime of fatigue critical components. Efforts have been made and continue to be made to attenuate or eliminate these vibrations in order to extend the operational life of the aircraft.
The problem of hubshear vibrations in a helicopter rotor has been addressed in U.S. Pat. No. 3,540,809 to Paul et at. Inplane rotor vibrations are absorbed by means of bifilar vibration dampers which rotate with the helicopter rotor. The bifilar dampers oscillate with the rotating mast and are tuned to absorb the vibrations generated at a particular frequency. Further vibration absorber devices are disclosed in U.S. Pat. No. 3,372,758 to Jenney and U.S. Pat. No. 3,035,643 to Kelley et. al.
Despite the development of the above vibration absorbers there exists a need for a grip-mounted, tunable, free pendulum for damping inplane vibrations.